This invention relates to a method for fabricating the hub of a bicycle wheel, an apparatus for implementing the method and the bicycle wheel hub obtained by means of the said method.
The Applicant has recently conducted various studies and tests to make bicycle components, particularly bicycle wheel hubs, especially for spoke wheel competition bicycles, using structural fiber based material, typically carbon fiber based material. The advantage offered by this type of material is that of being lighter in weight with respect to the metallic materials used in the past given equal structural characteristics. Making a hub out of a single part of carbon fiber based material was difficult, at least utilising the technologies available at that time, due to the typical conformation of the bicycle wheel hub of the type described above. The hubs used in modern bicycle wheels present a complex cylindrical shape, with a central constant diameter section and two bell-shaped end sections with a wider diameter or other, even more complex, shapes. Additionally, it is desirable for the hub thickness to progressively increase from the central section towards the hub ends, so to ensure the necessary resistance characteristics in all areas of the hub, particularly on the ends, where the wheel spokes are anchored, while ensuring minimal weight at the same time.
The need of making a tubular body with the complex shapes described above has made it impossible to make the hub from a single part of structural fiber material, such as carbon fiber material.
The object of this invention is to overcome this technical problem.
In view of achieving this object, the invention provides a method for fabricating a bicycle wheel hub, characterised in that it comprises the following steps:
providing an expandable core,
applying a number of layers of structural fiber fabric incorporated in a plastic material matrix around the core to form a layered tubular body of predetermined shape and thickness around the core,
arranging the core with the layered tubular body formed thereon in the cavity of a mold,
increasing the temperature of the mold to a value sufficient to cause reticulation of the plastic material matrix,
expanding the core for applying a pressure on the tubular body inside the mold, and
removing the tubular body from the mold and from the core, so as to obtain a bicycle hub formed of a single piece of structural fiber material.
In a first embodiment, the expandable core is made of a synthetic material presenting a thermal dilatation coefficient exceeding 5xc3x9710xe2x88x9215 mm/xc2x0 C. and a maximum continuous heat resistance equal to at least 80xc2x0 C, the expansion of the core being obtained through the dilation of the material forming the core when the temperature of the mold is increased.
Preferably, in this embodiment, the material forming the core has a thermal dilation coefficient exceeding 9xc3x9710xe2x88x925 mm/xc2x0 C. and a maximum continuous thermal resistance temperature exceeding 100xc2x0 C.
Again preferably, the material forming the core can be either PTFE (polytetrafluoroethene), or FEP (fluorinated ethene propene), or PCTFE (polychlorotrifluoroethene), or PVDF (polyfluorodivinylidene), or PE-HD (high density polyethylene).
The use of PTFE is widely preferred, due to the anti-adherence properties of this material, which are useful for detaching the core from the structural fiber molded body, as well as its high continuous thermal resistance (260xc2x0 C.), for its good thermal conductivity (0.25 W/mxc2x0 C. ) and for its good thermal capacity (specific heat), equal to 1.045 kJ/kgxc2x0 C.
The method, which main phases are outlined above, can be used in general to make hubs of all shapes, also different from that described above. A highly preferred characteristic of this method is in the arrangement of the aforesaid core made of high thermal dilation synthetic material, preferably PTFE. This material presents the characteristic of being subject to high thermal dilation at relatively low temperatures, in the order of temperatures at which the plastic material in which the structural fiber fabric is incorporated reticulates.
In a second embodiment of the method of the invention, the expandable core includes a body of metal material covered with a deformable sheath made of an elastomeric material, the expansion of the core being obtained through the dilation of the material forming the sheath when the temperature of the mold is increased.
Preferably, in this embodiment, the elastomeric material forming the aforesaid sheath has a thermal dilation coefficient exceeding 15xc3x9710xe2x88x925 mm/xc2x0 C. and a maximum continuous heat resistance temperature exceeding 100xc2x0 C. Still preferably, this material is a synthetic rubber of the type marketed under the trademark AIRCAST 3700 by Airtech International Inc., Huntington Beach, Calif., USA.
According to a further preferred feature of the second embodiment, the sheath is pre-formed according to the configuration of the core and is dimensioned in order to be applied on the core by slightly stretching it, so that the sheath adheres to the core due to its elasticity.
In a third embodiment of the method of the invention, the expandable core includes a body of metal material without any deformable sheath. In this embodiment the metallic core is divided in sectors, which can be expanded by means of mechanical means or, if provided by elastomeric junctions, by means of a gas injected inside the metallic core.
Structural fiber fabrics incorporated in a plastic material matrix are known and have been used for some time. They are made with yarn obtained from structural fibers, such as carbon fibers, for example. These fabrics are then subjected to a calendering process to associate them to a plastic material matrix, typically a thermosetting plastic material.
In the method of the invention, the structural fibers are selected among carbon fibers, glass fibers, Kevlar fibers, or any combinations thereof.
According to another important feature of the invention, the layers of fabric on the core comprise one or more fabric strips wrapped around at least one axially limited portion of the core, to confer thickness to the tubular body, as well as a plurality of fabric plies extending along the core axis, to confer resistance in the axial direction to the tubular body.
Finally, this invention also relates to a bicycle wheel hub, characterised in that it has a tubular body made of a plurality of layers of structural fiber fabric incorporated in a plastic material matrix, said layers including one or more fabric strips wrapped around at least one axially limited portion of the hub body as well as a plurality of fabric plies extending along the hub axis.